In frequency converters for speed regulation of electric motors, pulse width and pulse amplitude modulation of power semiconductors is extensively used. A side effect of this modulation is the generation of conductor transmitted radio noise. The standard EN 55011 describes limit values for emission of radio noise for devices generating electric noise. The noise occurs typically in the range of several kHz up to about 30 MHz. Compliance with these limit values can be achieved by installing or connecting an RFI filter. When designing an RFI filter for a frequency converter, it should be taken into account whether the rectifier is controlled or uncontrolled, as likewise the switching frequency of the inverter should be taken into consideration. The length of the motor cables is also an important design parameter. RFI filters can be connected to the output of the frequency converter, installed in the intermediate circuit, or installed on the mains side.
However, the use of RFI filters causes problems when at the same time a fault current circuit breaker is installed on the mains side of the frequency converter. Heavy-duty RFI filters mounted on the mains side have one or more earthed (PE, Protective Earth) capacitors, which are so large, that the fault current circuit breaker is triggered during start up. RFI filters installed in the intermediate circuit can also cause activation of the fault current circuit breaker. This problem can be solved by using fault current circuit breakers with a higher trip current or by using fault current circuit breakers with delay circuits, so that pulses of short duration will not cause triggering.
Alternatively, the problem of unwanted triggering of the fault current circuit breaker can be solved by designing the RFI filter in such a way as to reduce the earth leakage current. For example, EP 0 930 695 describes an RFI filter connected to the mains side, which enables connection of a frequency converter to a fault current circuit breaker.
The RFI filter designer faces another problem, namely the use of symmetrically and asymmetrically earthed mains supplies. In symmetrically earthed mains supplies, the star point of the three phases is connected to earth, which is the prevailing convention in Europe. Asymmetric main supplies are often used in the USA, where one of the three phases is earthed, a so-called delta-earthed mains supply. Frequency converters on asymmetric mains supplies can cause considerable earth leakage currents, and will trigger a fault current circuit breaker more often than frequency converters connected to symmetric mains supplies. In our experience, connection of a frequency converter to an asymmetric mains supply occurs in approximately 10% of cases.
As previously mentioned, the noise-decoupling capacitors in the RFI filters cause inrush currents to earth. Another inrush current is that which runs through the intermediate capacitor during start up. Protection of the intermediate capacitor is described in JP 02155477 A, where a resistor is inserted in series with the intermediate capacitor. A relay is placed in parallel with the resistor, and the relay is held open by a controller during start up, and closed during operation.
SU 1658344 describes a frequency converter, which in its intermediate circuit contains two capacitors connected in series between the positive and negative conductor. From the midpoint of the capacitor, a PTC (Positive Temperature Coefficient) resistor is connected to earth. The function of the resistor is to limit the large start up current drawn by the capacitor. Because the resistance is very low during start up, the capacitor connected to the negative conductor can be regarded as short-circuited, thus limiting the charging current. As the temperature rises, the resistance becomes so great that its presence has no influence during operation. However, SU 1658344 does not describe the problems associated with the use of fault current circuit breakers in combination with frequency converters, and the PTC solution used in SU 1658344 will most probably result in such a large flow of current to earth, that the undesired triggering of a fault current circuit breaker will occur.
For the most part, known methods describe protection of the intermediate capacitor. In the Danish patent application PA 2000 01837, the applicant has described a circuit comprising an inrush resistor inserted in series with a noise-decoupling capacitor in the intermediate circuit, and a switch is connected in parallel with the resistor. This resistor attenuates the common mode current, i.e. the current flowing through both positive and negative conductors and thereafter to earth. The common mode current can consist of noise current from the inverter or so-called surge pulses from the mains supply. A second switch is connected to a second inrush resistor in series with two intermediate capacitors, and a switch is connected in parallel with the resistor. The purpose of the resistor is to attenuate the differential current during start up, i.e. the current that flows from the positive to the negative conductor. However, a disadvantage of the circuit is the requirement for two relays. The physical dimensions of these relays are large, which is undesirable since the tendency for frequency converters is towards ever-smaller motor controllers.
On the basis of the above, the object is to develop a protection circuit for the dc intermediate circuit, which protects both the intermediate capacitor and noise-decoupling capacitor, and requires less physical space than hitherto.